assume cs:code
code segment
start:
    mov ax, 2000h
    mov ds, ax
    mov bx, 0
s:
    mov cl, [bx]
    mov ch, 0
    jcxz ok
    inc bx
    loop s
ok:
    dec bx
    mov dx, bx

    mov ax, 4c00h
    int 21h
code ends
end start